The invention relates to making uniform fiber sheet by spreading out tows, such sheets being used in particular for making the reinforcement of composite material parts.
In order to make unidirectional fiber sheets of relatively large width, it is well known to spread out a plurality of yarns or tows and to juxtapose the spread-out yarns or tows. Spreading out can be performed by passing over curved bars (or “banana” bars) or by passing through an air spreader.
Unidirectional sheets made in that way can be superposed in different directions and bonded to one another, for example by needling, in order to form multidirectional sheets. They may also be bonded, e.g. likewise by needling, with other types of fiber fabric, in particular woven cloth, so as to constitute complex or laminated fiber fabrics that are essentially two-directional.
In order to reduce the costs of making fiber sheets, and in particular when the fibers are carbon fibers, it is desirable to use as raw material the thick yarns or tows that are commercially available and of price per unit weight that is significantly lower than that of thinner yarns.
Nevertheless, the Applicant has found that it is more difficult to make uniform sheets by spreading thick yarns or tows and juxtaposing the spread yarns or tows than when using thinner yarns. Discontinuities (overlaps or gaps) are difficult to avoid at the junctions between spread-out yarns or tows.